filter-branch: nearest-ancestor rewriting outside subdir filter

filter-branch: nearest-ancestor rewriting outside subdir filter

Since a0e4639 (filter-branch: fix ref rewriting with
--subdirectory-filter, 2008-08-12) git-filter-branch has done
nearest-ancestor rewriting when using a --subdirectory-filter.

However, that rewriting strategy is also a useful building block in
other tasks.  For example, if you want to split out a subset of files
from your history, you would typically call

  git filter-branch -- <refs> -- <files>

But this fails for all refs that do not point directly to a commit
that affects <files>, because their referenced commit will not be
rewritten and the ref remains untouched.

The code was already there for the --subdirectory-filter case, so just
introduce an option that enables it independently.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>